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New Study Attempts to Reveal Deep Secrets of the Sleeper Shark

Although most people think of the largest predatory shark as the great white shark, a lesser known shark might hold that distinction. Although some historical projections estimate the largest white shark at around 23 feet, the largest living white sharks have been scientifically verified at around 21 feet. The Pacific sleeper shark (Somniosus pacificus) has been verified at 23 feet, and may in fact grow event larger, making it the longest predatory fish in the ocean. What the white sharks lack in length however, they make up in girth (14 feet), and weight (as much as 5000 pounds).

Pacific sleeper sharks are found primarily in the North Pacific, on continental shelves and slopes in Arctic and in temperate waters between latitudes 70°N and 22°N. Once thought to be confined to the North Pacific, individuals have also been identified in Palau and the Solomon Islands in the western tropical Pacific. In 2015 a Pacific sleeper shark was filmed swimming at the base of an active volcano near the Solomon Islands.

Little is known about this slow moving, smiley-faced cold water shark. Most observations have come from specimens as unwanted bycatch on commercial fishing vessels. New genetic evidence suggests that the Pacific sleeper shark is one single, largely distributed stock in the whole Pacific Ocean. Previously. it was believed to consist of a complex of several species.

Observed declines in certain parts of its range, combined with its low reproductive levels, are leading to conservation concerns. The Pacific sleeper shark is also one of the most vulnerable of all managed fish stocks US waters. A new NOAA Fisheries study published this month provides increased information and a strategy toward better understanding and managing the Pacific sleeper shark. The study led by Dr. Beth Matta, a research fisheries biologist at the NOAA Fisheries Alaska Fisheries Science Center examined existing literature on Pacific sleeper sharks and their close relative the Greenland shark. The search uncovered many obscure records, some of them more than 100 years old.

This species has been found foraging in shallow intertidal zones, and sighted by submersibles at depths beyond a mile underwater. Large sleeper sharks estimated 23 feet have been captured on submersible camera footage. One unconfirmed estimate at 30 feet would place the shark third in largest behind the whale shark and basking shark. However, the largest Pacific sleeper shark caught and verified by size measured 14 ft long and weighed 1,958 lb.

Adult Pacific sleeper sharks are rarely encountered. Most Pacific sleeper sharks encountered by fishing vessels are juveniles, and no pregnant female has ever been retained. Radiocarbon aging of the eye lenses indicates the sharks grow relatively slow. A 14-foot deceased female caught in the Aleutians revealed that the shark was estimated at 35 years, and was still reproductively immature. Scientists believe that mature sharks may live in deep, abyssal habitats, from 9,842 feet to 19,585 feet.

Sleeper shark recorded by CI and University of Hawai’i scientists off Lo’ihi Seamount, around 1000 meters.

According to NOAA Fisheries, the species appears to be in decline off the U.S., prompting conservation concerns. This is due to its lack of commercial value, deep habitats, and the difficulty involved in safely landing and handling large sharks aboard fishing vessels. Very little is known about Pacific sleeper sharks, including how many exist, the extent of their range and movements, or where they reproduce.

Unlike many midwater sharks, Sleeper shark livers do not contain squalene, an oil commercially harvested from many species of sharks like the Pacific Spiny Dogfish. Squalene is a light organic oil that is used in lubricants, cosmetics and vaccines, creating concerns about overfishing. Instead, Sleeper sharks have low-density compounds in the sharks’ liver including diacylglyceryl ethers and triacylglycerol, which maintain their liquid nature in the cold temperatures of the abyss.

Knowledge gaps for Pacific sleeper sharks include longevity estimates, bioenergetics, movements, and reproduction. Like many other sharks, the Pacific sleeper shark likely grows slowly, matures late, and has a long lifespan and low productivity. These qualities make it highly susceptible to overfishing and have led to a Near Threatened status by the International Union for the Conservation of Nature. With hope, the study led by the NOAA team will lead to a better understanding and management of this enigmatic species.

Banner Image: Pacific sleeper shark photographed at 3,125 feet depth by the remotely operated vehicle, Deep Discoverer. Credit: NOAA Office of Ocean Exploration and Research.

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